Effects of ovarian steroid hormones on dopamine-controlled prolactin secretory responses in vitro

Dopamine (DA), a well-established inhibitor of prolactin (PRL) secretion, has also been shown to stimulate PRL secretion from the anterior lobe of the pituitary gland of the rat. It has been reported that low doses of DA stimulate PRL whereas high doses inhibit PRL secretion. Our laboratory has previously reported that these PRL secretory responses are dependent upon the stages of the estrous cycle of the rat. The objective of the present study was to determine the steroid requirements for the differing PRL secretory responses to low and high doses of DA in perifusion. Animals were ovariectomized (OVX) and immediately given Silastic implants containing estradiol (E2) which has previously been shown in our laboratory to produce blood levels of 70-100 pg/ml, progesterone which has previously been shown in our laboratory to produce blood levels of 30-40 ng/ml, or the combination. OVX rats served as controls. Ten days later, the anterior lobes of the pituitary glands were harvested and enzymatically dissociated. Cells were mixed with Sephadex G-10 and placed in six 0.5-ml perifusion chambers (1 x 10(6) cells/chamber). Cells were perifused for 24 h with Dulbecco's modified Eagle's medium containing 0.2% bovine serum albumin and 0.1 mM ascorbic acid. The PRL secretory pattern was characterized in response to the following treatment sequence: (1) 30 min media alone (maximally uninhibited); (2) 24 min 100 pM DA; (3) 30 min media alone (DA withdrawal); (4) 24 min 1 microM DA, and (5) 30 min media alone (DA withdrawal). PRL secretion in the presence of 100 pM DA was unchanged in cells obtained from OVX animals, but exposure to 1 microM DA inhibited PRL release in these cells. Subsequent withdrawal stimulated PRL secretion relative to that of the initial exposure to media alone. The responses of cells from OVX rats implanted with progesterone alone was indistinguishable from those of the OVX controls. In cells obtained from animals implanted with E2 alone, 100 pM DA and its subsequent withdrawal neither stimulated nor inhibited PRL secretion. In contrast, 1 microM DA exposure initially stimulated and then inhibited PRL secretion in cells from E2-treated animals. Here, subsequent withdrawal of DA enhanced PRL secretion. In cells obtained from E2+progesterone-treated animals, 100 pM DA exposure robustly enhanced PRL secretory responses. Withdrawal of this dose of DA had no further effect on PRL secretion. However, exposure of E2+progesterone-treated cells to 1 microM DA robustly stimulated and subsequently only slightly inhibited PRL secretion. The results of these studies suggest that inhibition of PRL secretion by DA is independent of ovarian steroids while E2 and progesterone in combination favors stimulation of PRL secretion in response to DA. Taken together, these data suggest that PRL secretory responses in this system are determined by the ovarian steroid milieu.